Generally, implanting dopants is a critical process step in the manufacturing of semiconductor devices that gives manufacturers a controlled method of changing the electrical characteristics of chosen regions within the semiconductor device. A typical ion implantation process uses an ion implanter to initially generate ions of the desired dopant and then accelerates these ions to an appropriate energy level. Once accelerated, the ion implanter then transports the ions along an ion beam to impact and implant into a semiconductor wafer.
However, because the ion beam does not typically cover the entire wafer at once, the illumination of the wafer by the ion beam is controlled by a wafer-manipulator which sweeps the wafer at constant speed across the ion beam which is anchored at a fixed position. These sweeps generally include a constant velocity implant with a number of back-and-forth motions separated by incremental advancements of the wafer occurring between each motion in one direction. Once the advancements have completed in one direction, the wafer is rotated (typically 90°) and another set of incremental passes are used with the wafer being advanced in a second direction relative to the ion beam. This causes any single point of the wafer to be included in multiple sweeps (from the first increment in which the ion beam illuminates the point and including each increment until the ion beam moves past the point), with the total ion concentration determined from the accumulation of ion implantations during each pass of the overlapping scans.
However, using a constant velocity implant that is controlled by a two-dimensional wafer manipulator (by performing one pass and then rotating the wafer for another pass of incremental implants) only allows for a two-dimensional control of the implantation process. This simple, two-dimensional motion control also fails to take into account the three dimensional topology of the wafer itself, which can adversely vary the doping profile of the wafer. Without such three dimensional control, the typical two-dimensional ion implanter cannot obtain a uniform functionality of the resultant semiconductor devices (e.g., drain-current vs. voltage characteristics, clock speeds, leakage currents, etc.) because it cannot take into account this third dimension.
What is needed is an ion implanter that can take into account variations in the topography of a wafer in order to obtain a uniform functionality across the wafer.